Continuous closed-circuit desalination method without containers

ABSTRACT

A method for continuous closed-circuit consecutive sequential desalination of a salt water solution by reverse osmosis without using containers. The closed circuit includes one desalination module or more than one desalination modules connected in parallel. The method provides for desalination to proceed in concentrate recycling mode with brief intervals of concentrate discharge mode, in which pressurized concentrate in the closed circuit is replaced by pressurized fresh feed of salt water solution without stopping desalination.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/630,297, filed on Dec. 19, 2006, now U.S. Pat. No. 7,695,614, thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus withoutcontainers for continuous consecutive sequential desalination in closedcircuit of salt water solution by reverse osmosis.

DESCRIPTION OF THE PRIOR ART

Conventional reverse osmosis (RO) is carried out by means ofhydrodynamic “plug flow” techniques, whereby, pressurized feed flow issplit continuously into permeate and brine. A different approach that ofhydrostatic desalination in closed circuit was first reported in thelate eighties by Szuz et al. in U.S. Pat. No. 4,983,301 and by Bartt inU.S. Pat. No. 4,814,086, with emphasis placed on energy saving.According to the Closed Circuit Desalination (CCD) approach, thepressurized feed is recycled under hydrostatic pressure to the desiredrecovery, then brine is replaced with fresh and the batch processrepeated. The aforementioned patents suggested the alternatingengagement of two tanks in the closed circuit in order to enablecontinuous desalination. The principle drawback of this approach arisesfrom the need of “two relatively large capacity tanks”.

Szuz et al refer to the removal of “foreign materials by membranefiltration equipment, and therefore, apply to RO as well as tofiltration of suspended particulate matter from solutions and fluids.Desalination or filtration in closed circuit involve a strong dilutioneffect during recycling, and therefore, facilitates the attainment ofhigh recovery under relatively mild conditions.

A recent patent application WO 2005/016830 describes apparatus forcontinuous CCD with a single container. The current patent applicationdescribes a new inventive apparatus and method for continuous closedcircuit desalination of salt water solution by a continuous consecutivesequential process without the need for containers.

SUMMARY OF THE INVENTION

The present invention proposes apparatus and methods for continuousdesalination of salt water solution or brackish water feed (henceforth“feed”) using closed circuit; wherein, concentrate is recycled by acirculation device through parallel modules comprise of one or moresemi-permeable RO membrane elements; pressurized fresh feed supplydevice to said closed circuit to replace released permeate, and thevalve device to enable brine released from said closed circuit at thedesired recovery level without stopping the desalination process.

The apparatus and method according to the present invention forcontinuous desalination by a consecutives sequential process, can bemade to operate under constant pressure of variable flux and flow ofpermeate, or under variable pressure of constant flux and flow ofpermeate as result of fixed Net Driving Pressure (NDP). The NDP, thedifference between applied pressure and osmotic pressure, at each stageduring the operation of the inventive apparatus is controlled above apredefined minimum value.

The apparatus and method according to the present invention can be madeavailable in a modular form with pressurized feed centrally created andsupplied simultaneously to more than one modular unit. The modular formof the apparatus may apply to desalination plants of any productioncapacity.

The apparatus and method according to the present invention can containcommercial components and parts, which are actuated without exceedingtheir stated specifications.

The apparatus and method according to the present invention arecharacterized by simple inexpensive designs with relatively few powercomponents, reduced number of membrane elements and low specific energydemand without any need for energy recovery. The inventive method may befound specifically attractive for high desalination recovery (about75%-95%) of low concentration brackish water, and in this context mayprovide a simple cost effective approach to the partial removal of Boronfrom SWRO permeates to the acceptable level (about <0.5 ppm).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic diagram of the inventive apparatus fordesalination in closed-circuit with five modules each of three membraneelements (M5E15 configuration) during the concentrate recycling mode, inaccordance with a preferred embodiment of the present invention.

FIG. 1B is a schematic diagram of the inventive apparatus fordesalination in closed-circuit with five modules each of three membraneelements (M5E15 configuration) during the concentrate discharge mode, inaccordance with a preferred embodiment of the present invention.

FIG. 2A is a schematic diagram of the inventive modular unit of theapparatus for desalination in closed-circuit with five modules each ofthree membrane elements (M5E15 configuration) during the concentraterecycling mode, in accordance with a preferred embodiment of the presentinvention.

FIG. 2B is a schematic diagram of the inventive modular unit of theapparatus for desalination in closed-circuit with five modules each ofthree membrane elements (M5E15 configuration) during the concentratedischarge mode, in accordance with a preferred embodiment of the presentinvention.

FIG. 3 is a schematic diagram of a plant made of five modular units ofthe inventive apparatus, each of 5 modules and 15 membrane elements, inaccordance with a preferred embodiment of the present invention.

FIGS. 4A and 4B are graphs depicting simulated module inlet and outletTDS concentrations, and permeate TDS concentrations respectively duringan exemplified high percent recovery CCD process in accordance with apreferred embodiment of the present invention.

FIGS. 5A and 5B are graphs depicting simulated module inlet and outletBoron concentrations, and permeate Boron concentrations, respectively,during the exemplified high percent recovery CCD process demonstrated inFIG. 4.

FIG. 6A is a graph depicting simulated module inlet and outlet Magnesiumconcentrations and FIG. 6B is a graph depicting the percent ofsolubility of Magnesium with respect to Ksp of Mg(OH)₂ during the sameexemplified high percent recover CCD process as of FIGS. 4 and 5.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the inventive RO apparatus in FIG. 1Acomprises a closed-circuit desalination system with 5 modules (M₁, M₂,M₃, M₄ and M₅) each of 3 membrane elements, feed inlet (F), feedpressurizing pump (PP), circulation pump (CP), brine effluent outlet(B), permeate outlet (P), a three-way valve (V3), solution conductivitymonitoring devices (CM(1) and CM(2)) and pressure monitoring devices(PM(1) and PM(2)). In FIG. 1A, the conducting lines for fresh feed,mixed with concentrate or not, are indicated by solid lines, theconducting line for brine removal by a dotted line and the conductinglines for permeate collection by dashed lines. The directions of flow inthe various lines are indicated by arrows. The volume of the closedcircuit is derived form the volume of concentrate contained in thepressure vessels, also known as modules, and the pipes and the desiredclosed circuit volume is designed with the appropriate selection ofdiameters of pipe lines. The configuration depicted in FIG. 1A is thatof the concentrate recycling mode which is experienced during most ofthe time in this inventive apparatus.

The continuous consecutive sequential desalination in the inventiveapparatus requires periodical replacement of brine concentrate withfresh feed in the closed circuit at the desired system recovery leveland this brief mode of operation is depicted in FIG. 1B. Brine rejectionfrom the closed circuit is achieved by diverting the concentrate flow tothe outside through the three-way valve V3. The control of the entiredesalination process is achieved by means of “on line” monitoredconductivity with a signal form CM(1) that manifests reaching thedesired recovery level deactivating recycling in favor of brinerejection; and likewise, a signal form CM(2) that manifests brinerejection completion reactivating the recycling mode.

The pressure monitoring devices PM(1) and PM(2) in the inventiveapparatus under review provide the means for pressure control especiallyduring the brine rejection mode (FIG. 1B), with a minimum pressurechange in the system attained by means of valve actuation in response toundesirable “on line” pressure variations.

The preferred embodiment of the modular unit of the inventive apparatusdisplayed in FIGS. 2 (A and B) contains no pressurizing means and thepurpose of the two-way valve V2 in this design is to enable theisolation of a specific modular units for maintenance and/or repairwithout stopping the operation of the remaining units in the array. Byanalogy with FIG. 1A and FIG. 1B, the functions described by FIG. 2A andFIG. 2B are of recycling and brine rejection, respectively.

The combining of five modular units of the preferred embodimentsdepicted in FIGS. 2 (A and B) into a desalination plant, with its feedpressurizing device, is displayed in FIG. 3. The five modular units(MU-1, MU-2, MU-3, MU-4 and MU-5) in FIG. 3 are centrally suppliedthrough the same conducting line (F) with pressurized feed received fromPP, and similar such conducting lines are used for the collection ofpermeate (P) and for the removal of brine (B). The preferred mode ofoperation of plants comprised of modular units by the inventive methodis that under constant pressure conditions, although variable pressureoperation can be made possible provided each of the modular units in thearray is also equipped with a variable pressure booster whereby thedesired pressure variations are discretely created per modular unit.

It will be understood that the design of the preferred embodiments ofthe inventive apparatus and modular desalination units shown in FIGS. 1(A and B), FIGS. 2 (A and B) and FIG. 3 are schematic and simplified andare not to be regarded as limiting the invention. In practice, thedesalination units and apparatus according to the invention may comprisemany additional lines, branches, valves, and other installations anddevices as necessary according to specific requirements while stillremaining within the scope of the inventions and claims.

All the preferred embodiments depicted in FIG. 1-3 display basicdesalination apparatus and modular units made of five modules (pressuresvessels) with three membrane elements per module, and this for thepurpose of simplicity, clarity, uniformity and the convenience ofpresentation. It will be understood that the general design according tothe invention is neither limited nor confined to five modules permodular unit and/or apparatus and that the number of membrane elementsper module is not confined to three. Specifically, it will be understoodthat apparatus and modular units according to the invention method maybe comprised of n modules (M(1), M(2), M(3) . . . M(n)). Likewise, itwill be understood that each module may contain according to theinvention method m membrane elements (1E, 2E, 3E . . . mE).

The scope of the invention is neither confined nor limited to the designand construction of desalination plants that comprise 5 modular unitsaccording to the preferred embodiment describe by FIG. 3. It will beunderstood that desalination plants according to the inventive methodmay be made of any desired number of the inventive modular units andthat such plants are also within the scope of this invention.

Feed pressurizing devices for apparatus and plants made of modular unitsaccording to the inventive method may be comprised of variable flowconstant pressure pumps, or of constant flow variable pressure pumps,depending on the desired mode of operation of the designed system. Itwill be understood that feed pressurizing devices according to theinvention may be comprised of a suitable single pump, or instead, ofseveral suitable pumps that are applied simultaneously in parallel.

Concentrate recycling through the closed circuit of the apparatus andmodular units of the inventive method is done by circulation systems. Itwill be understood that the circulation systems according to theinvention may be comprised of a suitable single circulation pump, orinstead, of several circulation pumps, applied simultaneously inparallel and/or in line.

It will be obvious to those versed in the art that the inventivedesalination method under review may be operated in modular units and/ornon-modular desalination apparatus of different designs, as alreadyexplained above in respect of the inventive apparatus and/or units, aslong as such apparatus and/or units comprise a closed circuit ofconducting lines with one or more parallel modules each with one or moremembrane elements; circulation systems; feed pressurizing devices;permeate collection lines; brine removal lines; conductivity monitoringdevices, and pressure monitoring devices

Advantages offered by the inventive method and apparatus are asfollowed: Simple designs without staging and inter-stage booster pumps.

Apparatus made only of durable commercial components and parts.

Apparatus actuated without exceeding specifications of components.

Method saves on pressurization means.

Method saves on membrane elements.

Method saves on energy without need for energy recovery.

Method saves on operational expenses and maintenance costs.

Method affords permeate of low salt content.

Method suitable for boron removal form SWRO permeates.

While the invention has been described hereinabove in respect toparticular embodiments, it will be obvious to those versed in the artthat changes and modifications may be made without departing form thisinvention in its broader aspects, therefore, the appended claims are toencompass within their scope all such changes and modifications as fallwithin the true spirit of the invention.

EXAMPLE

The preferred embodiment of the inventive method is exemplified with ahigh desalination recovery (85%-95%) of a typical SWRO permeate feedwith TDS of 391 ppm that contain B (1.2 ppm) and Mg (3.4 ppm). Thisexample illustrates, amongst others, the application of the inventivemethod for the reduction of Boron level in SWRO permeates to an exceptedlevel (<0.5 ppm) by international standards.

The exemplified apparatus is of the schematic design displayed in FIGS.1 (A and B) with five 8″ pressure vessels (modules) each of threemembrane elements. The performance specifications under Test Conditions(TC) of the low energy commercial type elements considered in thecontext of the exemplified apparatus are as followed: Production, 44m³/day; Feed, 2,000 ppm NaCl; Pressure, 150 psi; Net Driving Pressure(NP), 8,8 bar; Temperature, 25° C.; and Maximum Element Recovery (MER),15.0%. The exemplified apparatus is operated under variable pressureconditions of fixed NDP (8.8 bar) in the actual consecutive sequentialpressure range for 95% recovery of 9.1-12.3 bar. The consecutivesequential desalination period for 95% recovery is of 6.0 minutes andthe duration of brine replacement with fresh feed takes about 19seconds. The fixed modular recovery under the above specified operatingconditions is of 37.8% with an Average Element Flow (AEF) of 36.9m³/day, or 83.9% of the rated element flow under TC.

The closed circuit volume of the exemplified apparatus is 122 liters,the flow rate of the pressurizing pump (PP) is 23.1 m³/h and that of thecirculation pump (CP) 38.0 m³/h. The calculated specific energy for 95%recovery of the exemplified apparatus is 0.45 kWh/m³, assumingefficiency of 85% for both the PP and CP.

The desalination production rate of the exemplified apparatus is 23.1m³/h or 554 m³/day or 202,210 m³/year with rate of disposed Brine of 1.2m³/h or 29 m³/day.

The computer simulated performance of the exemplified apparatusdisplayed in FIG. 4-6 presume operation at pH˜10 with salt rejection of99.5% and boron rejection of 93.0%. Operation at pH>10 with greaterboron rejection is restricted by the presence of Mg, since its Mg(OH)₂derivative is highly insoluble (Ksp=1.2×10⁻¹¹ at 18° C.).

FIG. 4A shows a module inlet and outlet percent concentrations (TDS) asfunction of desalination recovery (maximum 95%) at pH˜10 in the exampleunder review and illustrates the strong dilution effect created duringclosed circuit desalination. The quality of permeates received in theexample under review as function of desalination recovery are projectedin FIG. 4B. Module inlet and outlet boron concentrations during theexemplified process are revealed in FIG. 5A and the quality of permeatesreceived in reference to boron displayed in FIG. 5B. Noteworthy are thepermeate mean boron concentrations 0.22, 0.24, 0.32 and 0.44 ppm in theexemplified process during desalination recoveries of 80%, 85%, 90% and95%, respectively, which are below the recommended level of 0.5 ppm.Module inlet and outlet magnesium concentrations during the exemplifiedprocess are revealed in FIG. 6A and the percent solubility products of[Mg⁺²] and [OH⁻] in reference to the Ksp of Mg(OH)₂, displayed in FIG.6B, reveal maximum of ˜24% which means that no magnesium hydroxideprecipitation occurs during the exemplified process.

1. A method for continuous consecutive sequentially closed circuitdesalination of salt water solution by reverse osmosis using at leastone closed circuit comprising one desalination module having an inlet, apermeate outlet and a concentrate outlet, or two or more desalinationmodules having their respective inlets and outlets connected inparallel, wherein said closed circuit comprises no containers, themethod comprising: continuously supplying a pressurized fresh feed ofsalt water solution to said at least one closed circuit at a pressuresufficient to enable effective reverse osmosis desalination andreplacement of released permeate by fresh feed of salt water solution insaid at least one closed circuit; recycling pressurized concentrate bymeans of at least one circulation system from the concentrate outlet tothe inlet of said one desalination module, or from the concentrateoutlets to the inlets of the two or more desalination modules, aftermixing with said pressurized fresh feed of salt water solution;continuously collecting permeate from said one or more desalinationmodules; monitoring progress of desalination in said at least one closedcircuit; upon detection of a desired desalination recovery level in saidclosed circuit, diverting flow of said pressurized concentrate outsidesaid closed circuit without stopping desalination until concentrate inthe closed circuit is substantially completely replaced by fresh feed ofsalt water solution; and upon detection that replacement of concentrateby fresh feed of salt water solution is complete, switching back toconcentrate recycling mode in which pressurized concentrate is entirelyrecycled from the concentrate outlet to the inlet of the onedesalination module, or from the concentrate outlets to the inlets ofthe two or more desalination modules, and released permeate is replacedby pressurized fresh feed of salt water solution; whereby, most of thetime desalination proceeds in concentrate recycling mode, in which onlyreleased permeate is replaced by fresh feed of salt water solution whilepressurized concentrate is recycled through the closed circuit, withbrief intervals of concentrate discharge mode, in which pressurizedconcentrate in said closed circuit is replaced by pressurized fresh feedof salt water solution without stopping desalination.
 2. The method ofclaim 1 wherein the concentrate recycling mode is performed with fixedpermeate flow under variable pressure.
 3. The method of claim 1 whereinthe concentrate recycling mode is performed with variable permeate flowunder constant pressure.
 4. The method of claim 1 wherein said saltwater solution comprises any one of the following: potable watersources; saline water sources; contaminated water sources; saline andcontaminated water sources; clear domestic effluents sources; clearindustrial effluents sources; clear effluents from cooling towers ofcentral air condition units; high boron SWRO permeates; or brackishwater sources with total salinity up to 8,000 ppm.
 5. The method ofclaim 1 wherein two or more closed circuits are combined by sharing thesame feed pressurizing device and the same conducting lines for permeatecollection and for brine effluent.